To date, for example, in a magnetic-field-winding electric rotating machine for a vehicle, a magnetic-field current is supplied from the brush to the magnetic-field winding of the rotor by way of the slip ring. In this situation, the brush slides being pressed by a spring mounted at the rear portion of the brush. Due to an electric loss and sliding friction in the sliding unit and respective electric losses in the energization paths thereof, heat is generated in the brush and the slip ring. The slip rings in pairs are provided separately at the positive side and the negative side; the brushes in pairs are provided separately at the positive side and the negative side. In general, the brushes and the slip rings are held in such a way as to be covered with an insulative resin for securing the electric insulation between the positive side and the negative side.
A labyrinth structure that prevents intrusion of external foreign materials and moisture and has a function of exhausting brush abrasion powder is provided between a brush holder containing the brushes and a slip-ring molding resin. A small gap is provided in the labyrinth structure; however, cooling air produced by a cooling fan provided on the rotor is scarcely supplied to a portion where the slip ring and the brush abut against each other. Due to these contributing factors, the temperatures of the slip ring and the brush rise high around the portion where the slip ring and the brush abut against each other. In this situation, when the temperature of the brush rises high, the wear rate becomes large, thereby causing the problem of decrease in the brush lifetime.
Accordingly, to date, a structure has been utilized in which radiating fins are provided outside the brush holder so that heat generated in the brush is radiated to the outside through thermal transfer (e.g., refer to Patent Document 1). Alternatively, a structure has been proposed in which the brush is held by a metal brush holder and a plate for supplying a magnetic-field current to the brush integrally molded with a brush holder base for holding the brush holder are connected with a bracket through the intermediary of a seal material so that heat generated in the brush is radiated to the bracket (e.g., refer to Patent Document 2). Further alternatively, a structure has been proposed in which a vent hole for cooling the brush and exhausting brush abrasion powder is provided in the brush holder so that the temperature of the brush is lowered and hence the brush lifetime is raised (e.g., refer to Patent Document 3).